In line with growing concerns about environmental issues, research into electric vehicles and hybrid electric vehicles which may replace vehicles using fossil fuels such as gasoline vehicles and diesel vehicles, one of major causes of air pollution, has been conducted.
Research into the use of lithium secondary batteries having high energy density as power sources of the electric vehicles and hybrid electric vehicles has been actively conducted, and some of the research are in a commercialization stage.
Lithium cobalt oxide (LiCoO2) is mainly used as a cathode active material of the lithium secondary batteries, and in addition, the uses of lithium manganese oxides, such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium nickel oxide (LiNiO2) are also in consideration.
Among these materials, lithium manganese oxides, such as LiMn2O4, have advantages in that thermal stabilities are excellent and the prices are relatively low, but may have limitations in that capacities are low, cycle characteristics are poor, and high-temperature characteristics are poor.
When considering the structure of LiMn2O4, lithium (Li) ions are located at tetrahedral sites (8a), manganese (Mn) ions (Mn3+/Mn4+) are located at octahedral sites (16d), and O2− ions are located at octahedral sites (16c). These ions form a cubic close-packed arrangement. The 8a tetrahedral sites share faces with the 16c octahedral sites having vacancies therearound to form three-dimensional channels, thereby providing the channels through which Li+ ions are easily move.
In particular, the biggest limitation of LiMn2O4 is that the capacity decreases as the number of cycles increases. This is due to a structural change known as the “Jahn-Teller distortion”, that is, a phase transition from cubic to tetragonal caused by changes in the oxidation number of Mn ion at the end of discharge (near 3 V). Also, the cause of the capacity fading may include a phenomenon of dissolution of manganese into an electrolyte solution.
In order to address these limitations, a significant amount of research into a method of excessively adding Li in an amount of 1.01 to 1.1 times the stoichiometric amount of Li to stoichiometric LiMn2O4 to prevent the site exchange between Li and Mn metal ions and a method of substituting Mn sites with transition metals or divalent and trivalent cations to control the oxidation number of Mn ion or prevent the phase transition from cubic to tetragonal has been conducted.
These methods may reduce the capacity fading in comparison to that of the stoichiometric LiMn2O4, but may not address the limitations such as the Jahn-Teller distortion and the dissolution of Mn2+.